European Journal of Soil Science, August 2013, 64, 466–475
doi: 10.1111/ejss.12038
Turnover and availability of soil organic carbon under
different Mediterranean land-uses as estimated by 13C
natural abundance
A . N o v a r a a,b , L . G r i s t i n a a , Y . K u z y a k o v b , C . S c h i l l a c i a , V . A . L a u d i c i n a a & T . L a M a n t i a a
a Dipartimento
di Scienze Agrarie e Forestali, University of Palermo, Palermo, 90128, Italy , and b Soil Science of Temperate Ecosystems,
Georg−August-University of Göttingen, Göttingen, 37077, Germany
Summary
Soil organic matter (SOM) is an important factor in ecosystem stability and productivity. This is especially
the case for Mediterranean soils suffering from the impact of human degradation as well as harsh climatic
conditions. We used the carbon (C) exchange resulting from C3 -C4 and C4 -C3 vegetation change under field
conditions combined with incubations under controlled conditions to evaluate the turnover and availability of
soil organic C under different land-uses. The 40-year succession of Hyparrenia hirta L. (C4 photosynthesis)
after more than 85 years of olive tree (Olea europaea L.; C3 photosynthesis) growth led to the exchange of
54% of soil organic C from C3 to C4 forms. In contrast, 21 years of vine (Vitis vinifera L.) growing after
H. hirta decreased the organic C content to 57%. Considering this exchange and decrease as well as the periods
after the land-use changes, we calculated the mean residence time (MRT) of soil C of different ages. The
MRT of C under grassland dominated by H. hirta was about 19 years, but was 180 years under the vineyard.
The rates of C accumulation under the H. hirta grassland were about 0.36 Mg C ha−1 year−1 . In contrast, the
rates of C losses after conversion from natural grassland to a vineyard were 1.8 times greater and amounted
to 0.65 Mg C ha−1 year−1 . We conclude that changes of land use from natural Mediterranean grassland to a
vineyard lead to very large C losses that cannot be compensated for over the same periods.
Introduction
Soil organic matter (SOM) consists of various heterogeneous
pools with different stability and turnover rates that play an
important role in soil fertility and carbon (C) sequestration (Ussiri
& Johnson, 2003). The level and composition of organic matter
pools in soil is determined by the equilibrium between the factors
affecting its formation and decomposition (Laudicina et al ., 2011).
Land-use and land-cover changes modify the turnover of C and
the formation of SOM (Dinesh et al ., 2003). Several studies have
shown that plant species differ in their capacity to modify soil
properties (Vinton & Burke, 1995; Cornelissen et al ., 2004). Thus
plant functional characteristics such as growth form, functional
traits, biomass allocation, tissue chemistry and lifespan can affect
significantly organic matter decomposition and nutrient dynamics
in the soil (Carrera et al ., 2009). The direction of changes, loss
or gain of soil C stocks after land-use change, depends also on
soil properties, climate and management (Novara et al ., 2012).
Correspondence: A. Novara. E-mail: [email protected]
Received 22 June 2012; revised version accepted 8 January 2013
466
In order to assess and predict the effects of land-use change
on C storage, quantitative knowledge of C stock magnitude and,
especially, understanding of mechanisms of SOM stabilization for
each specific environment are necessary.
Very few studies have focused on the effect of land-use change
on turnover and availability of C in Mediterranean ecosystems
(Gavrichkova et al ., 2010). Mediterranean areas in recent centuries
have been subjected to a substantial human impact, with intensive
cultivation altering the structure and functions of soil, leading
to erosion and degradation. Such strongly degraded lands have
been removed from agricultural use because of reduced economic
benefits and left for succession of (semi)natural vegetation
developing to ‘macchia’ and ‘garrigue’ vegetation (Loumou
& Giourga, 2003). Such semi-natural vegetation succession
contributes to improvement of soil properties over the longer
term and also changes the C stocks. Despite some studies on
C sequestration after natural vegetation succession in abandoned
land in other areas (Kurganova et al ., 2010), Mediterranean areas
are strongly under-represented.
To identify mechanisms controlling changes in C pools
associated with land use and land cover, various physical
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science
Turnover and availability of SOC 467
fractionation methods have been used (Jolivet et al ., 2003).
Physical fractionation studies of SOM have revealed that soil
aggregation, reducing accessibility of microbial biomass to
particulate organic matter, contributes to physical protection and
thus stabilization of SOM (von Lützow et al ., 2006). It was
observed that a greater OM content and greater mineralization
rates are usually associated with the macroaggregate fraction. On
the other hand, the OM associated with microaggregates may be
more protected physically and more recalcitrant biochemically
(Dorodnikov et al ., 2011). However, fractionation provides
information on the net change in C only and not on the
mineralization of organic matter originating from old vegetation
and the incorporation of plant residue from the new land use
(Shang & Tiessen, 2000). Alternatively, the turnover of SOM
can be evaluated with approaches based on natural differences in
δ 13 C of plants with C3 and C4 photosynthesis (Schneckenberger
& Kuzyakov, 2007). This isotopic approach permits calculation of
organic C pools derived from old (in most studies, C3 ) and new (in
most studies, C4 ) vegetation because the two types of vegetation
differ in their δ 13 C according to C3 and C4 photosynthetic
pathways (Werth & Kuzyakov, 2008).
Our study highlighted the effects of land-use changes on SOC
stocks and organic matter dynamics in Mediterranean ecosystems
using natural differences in δ 13 C of plants with C3 and C4
photosynthesis. In contrast to nearly all previous studies, we used
land-use changes in both directions (C3 → C4 and C4 → C3 ). A
unique research area with a history of vegetation change from a
C3 -olive (Olea europaea L.) plantation to a C4 -Hyparrenia hirta
L. grassland and from C4 -H . hirta grassland to a C3 -vineyard
(Vitis vinifera L.) was chosen to follow changes in both directions.
It is important to note that these vegetation changes occurred
at the same location with identical climatic conditions and the
same soil type. This is an important prerequisite for using δ 13 C
signatures to calculate mean residence time (MRT) of various
SOM pools, this being the spatial variation of soil δ 13 C in relation
to C input and soil texture (Bai et al ., 2012). H . hirta is widely
distributed in the Mediterranean area (Botha & Russell, 1988),
North Africa and the Middle East (Chejara et al ., 2008) from
sea level up to 600 m above sea level. It is a typical species of
degraded areas after disturbance such as fire or intensive grazing
with fast growth: this means that H . hirta grasslands are very
suitable for studying turnover and stabilization of SOM in semiarid
environments.
H . hirta produces much above- and below-ground biomass,
especially under water limitation as is common in Mediterranean
areas in summer. As is common for grasses, its contribution to
below-ground C input is greater than that of herbs, shrubs and
trees. Such accumulation and input of C into the soil under H .
hirta is, furthermore, better stabilized during periods of microbial
dormancy during summer. Moreover, natural grasses such as H .
hirta strongly decrease and even stop erosion, and consequently
contribute to maintenance of C stocks in the soil.
We hypothesize that such land-use changes in a typical
Mediterranean environment affect C dynamics in soil and the
natural recovery of soil fertility and functions. Our objectives were
to (i) assess the change in SOC stocks when agricultural land-use
is followed by native vegetation and vice versa, (ii) estimate SOM
turnover during secondary succession in an agricultural abandoned
land (C3 -C4 soil) and under an agricultural crop (C4 -C3 soil) and
(iii) reflect on the importance of various soil organic fractions in
the turnover and stabilization of SOM by common land uses in
the Mediterranean.
Material and methods
Study and sampling area
The study was carried out on the south side of Inici Mountain,
in Sicily, Italy (37◦ 57 45◦ N, 12◦ 51 34◦ E) (Figure 1). The area
is semiarid (Thornthwaite & Mather, 1955) with a typical
Mediterranean climate: most of the annual mean precipitation
(409 mm) falls between October and February; monthly average
temperatures range from 12◦ C (January) to 25◦ C (August).
The underlying geology is the result of tectonic overlaying
of various carbonate, carbonate-silicoclastic and terrigenous
geological bodies, which are formed in the upper Triassic–middle
Tortonian. The soil in the study area is classified as a Regosol
(WRB, 2006) with 60% clay, 25% silt and 15% sand.
A secondary succession was selected on a south-facing gentle
slope (2–5%) represented by an olive grove, H. hirta grassland
and a vineyard (Figure 1). The area was completely covered by
olives for at least 75 years until 1971, when a wildfire burnt
half of the olive grove area. The burnt area was left to undergo
secondary succession to H. hirta (C4 photosynthesis) grassland
and in 1990 half of the H. hirta grassland was converted to a
vineyard. The dates of the fire event and the vineyard planting
were determined using aerial photographs. Environmental factors
such as geological substrate, soil texture, slope, soil type and
aspect can be regarded as homogeneous for all the land covers
in the succession. Three soil samples were collected at 0–15cm depth for each stage of succession. To reduce the error
tolerance to less than ± 5%, 24 kg of soil was collected per
sample and mixed. The soil was air-dried and passed through a
2-mm sieve.
Soil analyses and aggregate-size fractionation
Wet aggregate-size fractions with no prior chemical dispersion,
were isolated by mechanical shaking of 100 g, air-dried fine earth
on a column with sieves of 1000, 250, 63 and 25 μm using
a Shaker AS 200 Sieve (RETSCH analytical, Haan, Germany)
(203-mm sieves, amplitude of 2 cm, frequency of 1.6 Hz and a
water flux of 2 litres minute−1 ). After the physical fractionation,
we distinguished four main aggregate-size fractions: 1000–2000,
250–1000, 63–250 and < 63 μm.
For δ 13 C analysis of CO2 -C, carbonates in NaOH were
precipitated with 1 ml 5.5 m CaCl2 (formation of a CaCO3
precipitate) and immediately washed/centrifuged three times with
degassed deionized water before being dried at 55◦ C for 24 hours.
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
468 A. Novara et al.
Figure 1 Schema of the experiment with three land-use types, secondary vegetation succession and periods of C3 and C4 residues input to study site in
Sicily (IT).
For each aggregate fraction, the relative mass distribution, the
C content and δ 13 C signature were measured. Soil OC content
was measured using an elemental analyser (NA1500 Carlo Erba,
Milan, Italy).
Soil C stock (Mg ha−1 ) was calculated as:
Cstock Mg ha−1 = BD × Ccon x D × CFcoarse ,
(1)
where Ccon is carbon content (%), BD is bulk density (Mg m−3 ),
D is depth thickness (m) and CF is a correction factor for stone
content (1- [gravel % + stone %]/100). Bulk density was measured
by using the volume of the collected sample and the mass of dry
soil in the sample (Blake & Hartge, 1986).
Incubation procedure
In the three soil land-use types (an olive grove [C3 ], H. hirta
grassland [C3 -C4 ] and a vineyard [C4 -C3 ]) C mineralization
dynamics was investigated on three sub-samples for each
replicate (nine samples) by incubating 10 g soil at 50% WHC
in 125-ml glass flasks at 25◦ C for 32 days. Soil respiration
rate was determined seven times (over 32 days), measuring
the CO2 accumulated in the headspace of the flasks by a gas
chromatograph equipped with a thermal conductivity detector
(TCD). Twenty-four hours before the CO2 sampling, all of the
flasks were ventilated with fresh air and then sealed with rubber
stoppers. Carbon dioxide sampling started 1 week after the start
of the incubation to allow the soil to equilibrate after handling
(Robertson et al ., 1999). The amount of CO2 evolved from each
flask was calculated according to Zibilske (1994).
Stable carbon isotopic analysis and calculations of old and
new C
The 13 C:12 C ratio of bulk soil, SOM pools and CO2 released
during incubation was measured using an EA-IRMS (elemental
analyser isotope ratio mass spectrometer Carlo Erba Na 1500,
model Isoprime (2006), Manchester, UK). The reference material
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
Turnover and availability of SOC 469
used for analysis was IA-R001 (Iso-Analytical Limited, Crewe,
UK, wheat flour standard, δ 13 CV-PDB = −26.43‰). IA-R001 is
traceable to IAEA-CH-6 (cane sugar, δ 13 CV-PDB = −10.43‰).
IA-R001, IA-R005 (Iso-Analytical Limited beet sugar standard,
δ 13 CV-PDB = −26.03‰) and IA-R006 (Iso-Analytical Limited
cane sugar standard, δ 13 CV-PDB = −11.64‰) were used as quality
control for the analysis. The C isotope results are expressed
in delta (δ) notation and δ 13 C values are reported in parts per
thousand (‰) relative to the Vienna Pee Dee Belemnite (VPDB)
standard.
Natural abundance of δ 13 C was used to determine the proportion
of C in SOM that was derived from the new crop and how
much C remained from the previous crop in each soil aggregate
size fraction. These proportions were calculated with the mixing
equation (Gearing, 1991):
New carbon derived (Ncd ) =
δ 13 Cnew − δ 13 Cold
13
δ Cbiomass new species − δ 13 Cold
In the second approach, the amount of total C mineralized
was calculated by the linear interpolation of two neighbouring
measured rates and the numerical integration over time as reported
in the following equation:
CO2 − C =
n i
(ri + ri+1 ) ×
d
d
+ . . . + (rn−1 + rn ) ×
2
2
(5)
where i is the date of the first measurement of CO2 -C rate, n is the
date of the last measurement of CO2 -C rate, r is the CO2 -C rate
expressed as mg CO2 -C kg−1 dry soil and d is the number of days
between the two consecutive CO2 rate measurements (3, 4, 4, 7,
4, 7 and 7 days corresponding to the days after the incubation start
of 3, 7, 11, 18, 22, 29 and 36, respectively). The C mineralization
rate was expressed as mg C g−1 SOC day−1 and was fitted to the
following first-order decay function:
Mineralized C = Cr e−kt
(2)
(6)
where Cr is the readily mineralizable C at time zero (the intercept
value), k is the decay rate constant and t is time.
and
Old Carbon derived (Ocd ) = 1 − Ncd
(3)
where Ncd is the fraction of C derived from new vegetation
(H . hirta and/or vines), δ 13 Cnew is the isotope ratio of the soil
sample, δ 13 Cbiomass new species is the isotope ratio of the colonizing
species (−13.3 ± 0.15‰ for H . hirta and 28 ± 0.09‰ for vines)
and δ 13 Cold is the isotopic ratio of the previous vegetation type.
The δ 13 C values of soil under H. hirta are different from δ 13 C of
H. hirta biomass. The portion of old C derived under vines does
not correspond to soil C4 -C amount. Under the vineyard, SOC
contains residue of C3 -C from previous land use (olive grove).
Therefore, we calculated the percentage of C3 -C and C4 -C of the
old C derived using the proportions of C3 -C and C4 -C under the
previous land-use.
Estimation of C turnover
The C turnover in the succession of natural vegetation was
estimated with two approaches: (i) δ 13 C isotopic signature shift
in SOM after C3 -C4 and C4 -C3 vegetation change (Werth &
Kuzyakov, 2008) and (ii) C mineralization during incubation
experiments (Blagodatskaya et al ., 2011).
For the first approach, the turnover of SOM (mean residence
time in years, MRT) was determined as a reciprocal of the rate
constant (k) of first order decay (Equation (4)) (Balesdent &
Mariotti, 1996; Derrien & Amelung, 2011):
k = − ln (1 − Ncd) /years since disturbance.
(4)
The MRT for C in soil under H . hirta and the vineyard was
calculated as the weighted mean MRT of new and old SOM. The
mass of new C added by plants to the soil was calculated both for
bulk soil and for each aggregate-size fraction.
Calculations
Corrections for dilution by atmospheric CO2 in the incubation jars
were made with the following equation:
δ 13 CO2 measured = f + δ 13 CO2 atm + (1 − f) × δ 13 CO2 sample , (7)
such that:
δ 13 CO2 sample = δ 13 CO2 measured –f × δ 13 CO2 atm
(8)
where f is the fraction of the sample value contributed by atmospheric CO2 , which is calculated using a background concentration of 450 μl l−1 CO2 in the incubation jars. δ 13 CO2 measured
is the measured isotopic ratio, δ 13 CO2 sample is the undiluted isotopic ratio of microbial respiration, and the δ 13 CO2atm
is the isotopic ratio of measured atmospheric (laboratory
air) CO2 .
Fractionation was determined solving the following equation
system for x :
[CO2 ]m × δ 13 Cm = x × [CO2 ]O × δ 13 CO + (1 − x)
× [CO2 ]H × δ 13 CH
[CO2 ]m = x × [CO2 ]O + (1 − x) × [CO2 ]H
(9)
where x is the olive soil’s CO2 production, [CO2 ]m is the
measured CO2 , [CO2 ]O is the olive soil CO2 and [CO2 ]H
represents the H . hirta soil CO2 . Values of −27.5, −13.3‰ and
−28.0‰ were used as δ 13 CO , δ 13 CH and δ 13 CV , respectively. The
equivalent procedure was carried out for the second succession
step (H . hirta compared with vineyard).
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
470 A. Novara et al.
Statistical analysis
The data for total soil C, new crop-derived C, C stock and content
for each fraction were analysed by analysis of variance (anova)
after assessing variance homogeneity. Differences between the
means were tested with the LSD test at P < 0.05. Standard errors
for regression parameters were calculated. SAS statistical software
was used (SAS Institute, 2001).
Results
SOC stocks in bulk soil and aggregate fractions
Carbon content during secondary succession increased by more
than 39% after conversion from an olive grove to H . hirta
grassland. In contrast, the conversion of natural grassland to a
vineyard after 21 years resulted in an SOC depletion of 32%.
Among the land uses, the largest SOC content was found under
H . hirta (25 ± 3 Mg ha−1 ), followed by the olive plantation
(18 ± 5 Mg ha−1 ) and vineyard (17 ± 3 Mg ha−1 ), showing a
strong decrease in C content in the bulk soil under cultivation.
The aggregate fraction < 63 μm was the most abundant because
of a large clay content: this was followed in order in the
63–250, 250–1000 and 1000–2000-μm fractions (Figure 2). In all
fractions, statistically significant differences were found between
H . hirta grassland and agricultural land use (olives and vines),
except for the 250–1000-μm fraction. Under all land uses, the
mass contribution of each aggregate size fraction to the bulk
soil increased with the decrease in aggregate sizes. However, no
significant differences were found for the two biggest aggregate
fractions (Figure 2).
In the olive grove and vineyard the largest C content was
recorded in the 63–250 μm fraction (12.1 and 14.1 mg C g−1 ,
respectively) while the largest content under H. hirta was measured in the 1000–2000-μm fraction (15.2 mg C g−1 ) (Figure 3).
On average more than 50% of bulk SOC was contained in the
smallest fraction (< 63 μm) because of the combined effects of
the largest amount of fine aggregate fractions in the soils and its
greater C content.
Figure 3 Carbon content in aggregate size fractions in soil under the olive
grove (black columns), vineyard (white columns) and H. hirta grassland
(grey columns).
δ 13 C signature in bulk soil and aggregate size fractions
δ 13 C values of bulk soil and aggregate size fractions were
significantly (P < 0.05) affected by land use, and were largest in
H . hirta grassland (C3 -C4 soil), followed by the vineyard (C4 -C3
soil) and the olive grove (C3 soil) (Table 1 and 2). Under each
land use, the differences among the fractions were small, except
in the case of the smallest. The coarsest fraction had similar δ 13 C
values to that of plant residues, while in the smallest fraction the
δ 13 C value became less negative for cultivated C3 soil and, in
contrast, more negative in natural grassland. The average δ 13 C
values among the aggregate fractions were 0.73, 1.23 and 0.48‰
for the olive grove, H . hirta grassland and vineyard respectively.
The effect of land-use change (from C3 to C4 and from C4 to
C3 ) on C stock was demonstrated by C derived from H . hirta and
from vines (Table 1). After 40 years of H . hirta grassland, 80% of
the C still originated from the olive grove, whereas in the vineyard
only 6% of the C had a H . hirta origin after 21 years. The landuse change from olives to H . hirta led to an increase of 2.7%
of C4 -C, but the stock of C derived from olives (C3 ) was mostly
preserved. In H . hirta grassland most of the C3 -C was contained
in the smallest aggregates and therefore this portion was stable
(Figure 4). The shift from H . hirta to vines led to the loss of
2.08 g kg−1 of C3 -C and 1.8 g kg−1 of C4 -C (Figure 4). Most of
the C3 -C loss when vines followed H . hirta was recorded in the
biggest aggregates.
Fluxes and sources of CO2 by incubation
Figure 2 Mass of aggregate size fractions in soil under the olive grove
(black columns), vineyard (white columns) and H. hirta grassland (grey
columns).
During 32 days of incubation, the largest CO2 emission rate was
recorded from the olive grove soil and ranged between 47 mg
CO2 -C kg−1 day−1 (first day of incubation) and 14.7 mg CO2 -C
kg−1 day−1 (after 32 days). This was followed by the soil under
vines (from 37 to 13.05 mg CO2 -C kg−1 day−1 ) and H . hirta
(25.05 to 8.86 mg CO2 -C kg−1 day−1 ) (Figure 5). The total C
mineralized from soil over 1 month was greater under the vineyard
and olive grove (592 ± 162 and 535 ± 82 mg kg−1 , respectively)
than in the H . hirta soil (383 ± 27 mg kg−1 ). The δ 13 C of the
CO2 evolved during the incubation ranged between −26.5‰ and
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
Turnover and availability of SOC 471
Table 1 δ 13 C (‰) and portion of C3 -C and C4 -C in bulk soil and aggregate size fractions
δ 13 C
Bulk soil
1000–2000 μm
250–1000 μm
63–250 μm
< 63 μm
Carbon distribution
Olive grove
H. hirta
Vineyard
H. hirta
Vineyard
C3 soil
C3 -C4 soil
C4 -C3 soil
C3 -C
C4 –C
C4 -C
C3 -C
-27.14
-27.14
-27.71
-27.16
-26.98
-24.37
-24.59
-24.58
-23.97
-25.20
-26.81
-26.81
-26.89
-26.67
-26.41
0.20
0.18
0.21
0.23
0.13
0.80
0.82
0.79
0.77
0.87
0.064
0.063
0.069
0.074
0.072
0.936
0.937
0.931
0.926
0.928
Table 2 Summary results for SOC (a) and δ13C from anova (b)
(a) Source
Degrees Sum
Mean
freedom squares square
F ratio P
Land use
2
1.8835 0.94176 4.02
Replicas
2
0.1520 0.07602 0.32
Aggregate fraction (residual) 428
2.0918 0.52295 2.23
Degrees Sum
Mean
(b) Source
freedom squares square
F ratio
0.02
0.7
0.09
Land use
2
Replicas
2
Aggregate fraction (residual) 428
0.00
0.02
0.04
69.8152 34.9076
3.9760 1.9880
0.1843 0.0460
72.96
4.16
0.10
P
−25.0‰ in the olive grove, from −26.0‰ to −23.5‰ in the
vineyard soil and from −24.0‰ to −21.5‰ in the H . hirta soil
(Figure 6). The δ 13 C values of CO2 from all soils were strongly
depleted during the first week of incubation. The contribution of
recent C4 to CO2 flux increased from 65% at the beginning to 91%
at the last stage (32 days) of incubation in C3 -C4 soil (Figure 7),
while it ranged from 40 to 84% in C4 -C3 soil. The ratio of C3 -C
in evolved CO2 to that in SOM was 0.37 for C3 -C4 soil and 0.21
for C4 -C3 soil (Figure 8).
Carbon turnover
Using the first approach, MRT of bulk soil was estimated as 19
and 183 years for H . hirta and olive grove SOM, respectively.
The MRT increased with decreasing aggregate size under both
land uses. In the fraction < 25 μm the MRT was 37 and 193 years,
respectively, for the H. hirta and olive grove soils. The MRT
calculated by the second approach, based on released CO2, was 30,
17 and 22 days for H . hirta , olive and vineyard soils, respectively.
These values reflect the MRT of easily available C in soil that was
involved in decomposition within 1 month.
Discussion
Land-use change and C stocks
SOC content was very sensitive to land-use changes. Forty years
after the abandonment of the olive grove and subsequent
natural vegetation succession, the OC in bulk soil increased
Figure 4 Content of C3 -C (white histograms) and C4 -C (black histogram)
in bulk soil and for each particle size-fraction in H. hirta grassland and
vineyard soils.
to 7 Mg C ha−1 . The opposite land-use change from natural
vegetation to agriculture, resulted, after 21 years, in a decrease
of 8 Mg C ha−1 . Our results confirm that agricultural use strongly
modifies soil properties and especially C content (Yan et al .,
2012): there is an increase in C content under natural vegetation
and less mineralization than in agricultural soil (Novara et al .,
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
472 A. Novara et al.
Figure 5 CO2 efflux rates over 32 days of incubation for the H. hirta
grassland, olive grove and vineyard.
Figure 7 Relative contribution of C4 -C (white dot) and C3 -C (black dot)
soil during 30 days of incubation of C3 -C4 soil (a) and C4 -C3 soil (b) from
H. hirta grassland and vineyard, respectively.
13
Figure 6
C signature of the evolved CO2 -C during 32 days of
incubation of C3 -C4 (white dots) and C4 -C3 (black dots) soils from the H.
hirta grassland and vineyard, respectively.
2013). Soil management with continuous tillage of the vineyard
and the olive grove in our study, contributed to the removal of
spontaneous vegetation growth and more OM mineralization as a
consequence of increased aeration, breaking aggregates (Barbera
et al ., 2012) and making more OM accessible to microbial activity
(Solomon et al ., 2002). As expected, the increase in C stock
under natural vegetation was less than the C depletion after the
vineyard plantation (Lal et al ., 2004). The rate of C increase after
abandonment of the olive grove was 0.36 Mg C ha−1 year−1 , while
the C depletion rate was 0.65 Mg C ha−1 year−1 . These rates of
C accumulation and losses in Mediterranean land use are faster
than those described for temperate, boreal and steppe climates
(Kurganova & de Gerenyu, 2008). The greater C depletion rate
compared with the C increase results from intensive soil tillage
in the vineyard that can also contribute to C losses by increasing
erosion.
Land use affected soil aggregate formation and destruction.
In agricultural soil the aggregates are destroyed by cultivation
(An et al ., 2010), resulting in an increase in the amount of
microaggregates. Soil under H. hirta had fewer microaggregates
than the agricultural soil. This demonstrates that over 40 years,
vegetation change and the consequent SOM increase strongly
contribute to formation of macroaggregates. (Majumder et al .,
2010). The distribution of SOC between aggregate size fractions
in this Mediterranean soil was similar to the pattern observed
previously in Sicilian clay soil (Barbera et al ., 2010; Novara et al .,
2012).
Land-use change from olives to H. hirta encouraged C
stabilization in microaggregates and increased C content in the
largest fraction through more litter production under H . hirta
grassland and subsequent containment in macroagregates. The C
stabilized in the smallest aggregates under H . hirta remained after
21 years of vineyard plantation, but C losses were noted in the
largest aggregates. This is connected with the decreases in plant
residues under vineyard management that are mainly allocated to
the large aggregates (Dorodnikov et al ., 2009).
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
Turnover and availability of SOC 473
Figure 8 Comparison between the relative contribution of C4 -C (grey
portion) and C3 -C (white portion) to SOM and CO2 in C3 -C4 soil (H.
hirta) and C4 -C3 soil (vineyard).
Stabilization of SOC in soil aggregate size fractions
Separation of SOC into aggregate size fractions in conjunction
with the use of the natural 13 C abundance helped us to follow the
mechanisms of SOM accumulation and turnover after olive groveH . hirta grassland-vineyard succession. Before the fire (40 years
ago), the OM input over a long period was solely of C3 origin.
Following the C4 vegetation succession, bulk soil and all fractions
were enriched in 13 C, showing the decay of C from olive origins
and H . hirta C accumulation. After 40 years of natural vegetation
development, 80% of C derived from the previous crop remained
in the soil and most of this was in microaggregates (< 63 μm)
(Figure 4). This indicates that the smallest fraction contained more
preserved OC and retained C derived from previous vegetation
types for longer and more efficiently than the coarser fractions
(Desjardins et al ., 2006), through physicochemical stabilization.
After the land-use change from H . hirta grassland to vineyard,
most of the C losses were as C3 -C and more than 30% of this C3 C was lost from the largest aggregates (Figure 4). These results
clearly show (i) faster decomposition of new C3 -C compared with
C4 -C derived from H . hirta and (ii) that there was not enough
litter input to soil to restore the previous C content of the biggest
aggregates.
CO2 fluxes and their δ 13 C signature during incubation
The ability of uncultivated soil to retain C was confirmed by CO2
fluxes over 32 days of incubation. The cumulative CO2 efflux in
H . hirta soil was 32 and 40% less than that from the vineyard and
olive grove soils, respectively (Figure 5). The δ 13 C signature of
evolved CO2 -C was depleted for all soils, but it was greater for C3 C4 and C4 -C3 soil than for C3 soil. Because the C3 soil reached the
isotopic steady-state after 85 years (Blagodatskaya et al ., 2011),
the difference in δ 13 C between SOM and CO2 -C is related to 13 C
fractionation or preferential use of 13 C-depleted substances such
as lipids or lignin (Werth & Kuzyakov, 2010).
In the soils after the C3 -C4 and C4 -C3 vegetation change,
not only 13 C fractionation but also preferential use of recent
rather than old C contributed to δ 13 C enrichment in CO2 . This
means that less stabilized younger organic materials (having C4
or C3 signature for C3 -C4 and C4 -C3 soils, respectively) will
be preferred for microbial utilization and consequently contribute
strongly to CO2 evolution.
The relative contribution of C4 -C to the evolved CO2 was
always more than 60% in C3 -C4 soil, and increased during
incubation as in other studies (Blagodatskaya et al ., 2011).
Although the relative contribution of C4 -C to SOC was less than
the C3 -C contribution, this C4 portion of organic matter in the
H . hirta soil represents the new C, which was less stabilized
and therefore more available for microbial use. The increase in
the contribution of C4 -C during incubation can be related to
microbial activity. It has been observed in short-term incubation
studies that in the earlier stage of incubation the total microbial
biomass decreased because of disturbance and the absence of
new C inputs. After the initial period, the subsequent microbial
population uses metabolites remaining after the first stage and, in
this case, decomposition of microbial tissue led to an enrichment
of the respired CO2 -C (Blagodatskaya et al ., 2011).
The contribution of C4 -C to the total CO2 efflux in C4 -C3
soil was less than the C3 -C contribution in the first few days,
but increased during the incubation period. However, the relative
contribution of C4 -C was less in the C3 -C4 soil than in the C4 C3 soil because of preferential use of new C (C3 -C). The ratio
between C3 -C and C4 -C in CO2 was, for both land uses, less
than that in the SOM. The CO2 emission had therefore originated
mainly from H. hirta (Figure 8). The ratio between C3 -C in CO2
and C3 -C in SOM was similar in the two soils, whilst that between
C4 -C in CO2 and C4 -C in SOM was greater in C4 -C3 soil. The
C of OM derived from H. hirta vegetation was therefore more
degradable in C4 -C3 soil.
Turnover of recent and old carbon
The C turnover in soil under H . hirta was about 10 times faster
than in that under the olive grove. The reasons for this difference
are: (i) the biomass of H . hirta is younger than that from olives,
which was added to the soil more than four decades before,
and therefore is more easily mineralizable; and (ii) the SOC in
the olive grove was mainly stored in the smallest aggregates
and was therefore more stable. According to Equation (6), the
MRT of C decomposed to CO2 within 1 month was 30 days for
H . hirta soil and 17 days for olive grove soil. Results of MRT
showed differences between the two estimation approaches: the
MRT of H . hirta was less in the first approach and more in
the second approach than that of the olive grove. The main
difference is that the first approach, using δ 13 C isotopic signature
shift in SOM after C3 -C4 vegetation change, estimates the MRT
value only of C4 organic matter, while the second approach
estimated the MRT of C3 -C4 soil under H . hirta. If only the
portion C4 -C of total SOC in H . hirta is considered, the MRT
© 2013 The Authors
Journal compilation © 2013 British Society of Soil Science, European Journal of Soil Science, 64, 466–475
474 A. Novara et al.
is approximately 6 days and therefore less than that for the olive
grove soil.
Conclusion
This study highlighted the effect of land-use change on SOC
stock and OM dynamics in a Mediterranean area using natural
differences in δ 13 C of plants with C3 and C4 photosynthesis.
We demonstrated that (i) intensive agriculture (olives and vines)
in the Mediterranean region has a negative impact on C
sequestration when compared with semi-natural vegetation; (ii)
vegetation change to H . hirta and consequent SOM increase
strongly contribute to the formation of macroaggregates from
microaggregates and (iii) turnover of SOM under H . hirta was
quicker than under olives, but soils under natural vegetation
emit less CO2 and therefore are able to sequester more SOC.
Consequently, we strongly recommend sustainable management
practices to avoid SOM decrease and soil erosion. We suggest the
application of organic fertilizers as well as promoting the growth
of natural grasses in the winter period or burying pruning residues
to contribute to C input and decrease erosion.
Acknowledgements
This work was financially supported by the PRIN project ‘The
impacts of secondary succession processes on carbon storage in
soil and biomass and on biodiversity and the role of dispersal
centers and vectors for recolonization processes’. We thank F.
Fiordilino for technical support during the research.
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